worthy -- thanks for replying... unfortunately, I can not slide in the XPS behind the studs. By local code, we had to install fire blocking on the end of the wall. So now it is closed off. And, I can't install XPS in between the studs becuase I already have electrical and piping running through the studs. The gap between the wall and the stud is only a little more than 1/2". So even if I could slide in the XPS behind the studs, I would only get about r 3 value.... Thats why I figured it is almost better to put no insulation. The pipes arent gonna freeze; Plus, if I do get moisture back there, then there will be no fiberglass insulation to get wet. With no insulation, the wall will just dry out. (with the help of my dehumidifers and the bathroom vent fan, of course) If I get a chance, I will post a picture tonight of the studded wall with the roughed-in plumbing and electrical. thanks again. mm I'll try to post a picture later....

If you buy faced insulation the kraft backing should go firmly against the conditioned side. On the other hand you could use the unfaced type there like the previous poster said. Either way you will still see the insulation from below. If you don't do this it may, or may not, come back to bite you one day.

I've used Warmly Yours in my foyer and a down stairs bathroom. It works well. I used hot glue to tack the mat to the floor. Then skimmed a light coat of thin set. The next day, I used unmodified thin set and put Ditra over the Warmly Yours mat. I would use Dirta because it's an anti-fracture membrane.

Curious to know if anyone has had experience with adding flooring to a basement that is both thin and insulating? Turning basement into ADU. Trying to get as much head heigh as possible: need to level floor, add thin rigid insulation and possibly adding warm boards for heat. Looking for suggestions of products and techniques.